I don't believe anyone is claiming the wall will stop 100% of the illegal border crossings. It will stop a lot though, not just as a stand alone, but it will slow crossers down making it easier for border control agents to intervene. It will also make it more difficult and expensive for the average would be illegal immigrant, thus acting as a deterrent to them even attempting to cross.
Mine fields in front of the wall and in back of the wall will get it pretty close to 100%. Just saying.